1. Field of the Invention
The present invention pertains to a method for measuring drillstem deflections and downhole drillstemcasing interaction and a system including an arrangement of strain gages and accelerometers for measuring vibrations, deflections and forces acting on the drillstem.
2. Background
In the drilling of oil and gas wells, it has been observed that severe wellbore casing wear has occurred to the point of unwanted penetration of the casing wall. In certain drilling operations unexplained vibrations and drillstem motions have also resulted in significant damage and failure of drillbits and other downhole portions of the drillstem beyond that which is explainable by sensing torque, rotary speed and weight on the bottom hole assembly.
Prior art efforts to develop instrumented drillstems have included the use of an instrumented assembly in the drillstem near the lower or bottom end thereof. However, this type of technique presents signal transmission problems and exposes the instrumentation to the pressures, temperatures and severe accelerations that occur at the lower end of the drillstem. Efforts have also been made to place devices such as accelerometers on the extreme upper end of a drillstem such as on the conventional swivel or drillstem supporting structure. Efforts have also been made to develop tools for measuring torque between a conventional rotary table and a drillstem. Such efforts have included the use of a radio transmitter to broadcast strain gage measurement signals. Accordingly, even though it has been contemplated to place sensing devices, including strain gages, at points along the drillstem below the surface and in proximity to the drillbit so as to measure total loading exerted on the bit as well as torsional, axial and lateral vibrations or deflections of at least portions of the drillstem, the operating environment in the borehole as well as the length of some drillstems tends to preclude the provision of a suitable service life for downhole instruments and complicates the transmission of signals to surface monitoring and recording devices.
There has been a long-standing need to provide a system for measuring the stresses and strains exerted on a drillstem so as to improve the service life of the drillstem, the bit, any downhole tools or motors used in the drillstem and to minimize wear on the drillstem and borehole structures such as metal casings which may be prematurely worn or damaged by engagement with the drillstem during severe loading or deflection thereof. Moreover, the collection and analysis of information regarding drillstem behavior in the vicinity of the bit or at other points along the drillstem below the surface can be useful in improving the bit penetration rate, the life of the drillstem, and to correct for operating conditions which may lead to premature failure or excessive wear on the drillstem or other wellbore structures.
Important goals in this regard include the elimination of excessive vibration induced casing wear, the quick identification of damaging bottom hole assembly vibrations, improvement in the performance of bottom hole assemblies intended to drill vertical as well as deviated or angle drill holes, and to provide a method for identifying and then eliminating vibrations that cause surface accelerations of the drillstem that mask the correlation between certain accelerations and deflections and occurrences in the hole which can be used to determine formation conditions or minimize unwanted failures of the drilling assembly. It is to this end that the present invention has been developed with a view to providing a method and system for measuring drillstem loading and behavior under various operating conditions.